The Pentax 645N (N II) is very well dampened and shutter shake is really not a problem.
Two images of moving aircraft on the taxiway taken with the (old) 67 300mm F4.0 on the P645N - both at 1/180 sec. and handheld:
Kramme & Zeuthen KZ II
Eurofighter Typhoon
and a commander of the Kravat Regiment in Zagreb taken with the 200mm at 1/60 sec. handheld - not much shuttershake there:
An here is an image taken with the Pentax 6x7 and the 6x7 500 mm F5.6 - 1/500 sec. handheld - maybe not super sharp, but a shot taken on the spur of the moment with the whole rig handheld, and the combo is quite heavy: